Biomedical Engineering Reference
In-Depth Information
Fig. 11.4
Typical mass
spectrum for collisions of 60
keV C5
with isolated
thymine molecules (grey) and
of 50 keV O
5C
with neutral
thymine clusters (black) [
49
]
C
observed in ion-induced desorption studies on thymine thin films, too [
20
], i.e. it is
likely that OH loss is a fingerprint of a chemical environment. Similar additional
fragmentation channels were also observed for clusters of other nucleobases. It
is concluded that these channels are probably due to intermolecular hydrogen
bonding between O and H atoms of neighboring thymine molecules that weaken
the intramolecular bonds. Similar experiments have recently been performed for
clusters of amino-acids [
50
]. Also here, the chemical environment drastically alters
fragmentation pathways due to rapid energy and charge redistribution inside the
cluster. In contrast to nucleobase clusters, where ion impact leads to formation
of nucleobase fragment ions or nucleobase cluster ions, for amino acid clusters
also polymerization is observed. In particular, loss of single or multiple units of
mass 18 are observed, which could be explained by stabilization of peptide bonds
within the cluster [
50
]. Note that this finding is of great interest in the context of
astrochemistry and astrobiology, since it hints at a possible pathway to formation of
complex biomolecules in astrophysical environments. In the context of biomolecular
radiation damage, however, the important outcome of the performed experiments is:
Ion-induced biomolecular fragmentation is strongly influenced by the presence of a
chemical environment. It is thus neccessary to study this influence in more details.
11.2.3
Biomolecules on surfaces
A complementary approach to gas phase studies is the investigation of ion-
interactions with thin biomolecular films. Such studies are appealing for a number
of reasons: First of all, in chromatin DNA is typically found in a relatively closely
packed structure, which in certain aspects resembles the solid phase. Furthermore,
interactions of biomolecules with a chemical environment are inherently included.
Last but not least, targets are dense and relatively easy to produce. On the other
hand fragment charge states and kinetic energies can change during the desorption
process, rendering the observed data to some extent indirect. Over the last years,
surface studies have contributed greatly to the knowledge on very low energy
ion-biomolecule collisions. Huels and coworkers could show that Ar
C
ions with
kinetic energies exceeding 10 eV can induced fragmentation of the nucleotide
